Hitherto, carbon black is employed widely as a filler for reinforcing plastics or rubbers.
In recent years, high properties such as high abrasion resistance are required for rubbers, and thus carbon black having a large specific surface area and high reinforcing ability has been blended with rubbers.
However carbon black having a large specific surface area are difficult to bond with a rubber component in rubber compositions, and thus the carbon black particles tend to aggregate with each other. As a result, it is not easy to uniformly disperse carbon black into the rubber component and under ordinary processing conditions, the desired reinforcing effect cannot be obtained. Also, if it is tried to disperse carbon black uniformly in rubber, a large amount of energy for kneading is required, which lowers productivity.
Particularly in the field of tires, low fuel consumption (low loss tangent) is required. If carbon black having a large specific surface area is blended with rubber to improve the mechanical strength of tires, the loss tangent (tan .delta.) of the tire becomes large and fuel consumption is increased.
For improving dispersibility of the carbon black having a large specific surface area into a rubber component, a method for modifying the surface of the carbon black particles has been proposed and has been partially practiced.
For example, in JP-B-5643/1970, JP-B-24462/1983, JP-B-30417/1968 and U.S. Pat. No. 3,282,719, there is proposed that the aggregation force between the carbon black particles is reduced by the addition of oily materials or high molecular materials. According to the proposal, however, there is less effect in the case where the carbon black having a large specific surface area are used, and an improvement in the mechanical properties and low fuel consumption of the tire cannot be satisfied at the same time.
Also, in JP-A-250042/1987, JP-A-250073/1987, JP-A-152165/1989 and JP-A-284564/1989, for example, there is proposed that in view of polar groups such as hydroxyl group, carboxyl group and carbonyl group on the surface of carbon black particles having a large specific surface area, dispersibility into a matrix polymer is improved by surface-treating with a compound having an affinity for these polar groups.
According to the descriptions of JP-A-250042/1989 and JP-A-250073/1989, the surface of the carbon black particles is treated with an amine compound which is an antioxidant for rubber, and dispersibility of the carbon black is improved, and the mechanical properties of the obtained rubber is also improved to some extent. The affinity to the matrix polymer is, however, insufficient and, further, low fuel consumption required in the tire field is also insufficent.
According to the descriptions of JP-A-152165/1989 and JP-A-284564/1989, the carbon black particles are surface-treated with a saturated high molecular weight compound containing a functional group which has an affinity to the polar groups on the surface of carbon black, and it has been tried to improve the affinity to the matrix polymer through the saturated high molecular chain portion. However, these technologies utilize the chemical affinity of the saturated high molecular weight portion to the matrix polymer and there is no chemical bonding with the matrix polymer such as rubber. Therefore, the bonding strength with the matrix polymer sometimes becomes insufficent and, particularly in the field of tires, it is difficult to improve low fuel consumption (to decrease loss tangent).
Further, there is proposed to bond chemically a high molecular weight compound with the matrix polymer such as rubber in JP-A-100730/1994. However, the compound has a functional group only at one end of the molecular chain, and thus an improved effect cannot be obtained.
As a result of the inventors' investigation of the above-mentioned prior art and their intensive study, it has been found that the following properties are necessary for a surface-treating agent for carbon black, which is superior in reinforcing ability, dispersibility and processability and has strong bonding strength to the matrix polymer such as rubber.
(1) Having a functional group which interacts with the acidic functional group on the surface of the carbon black particles and can bond with the carbon black particles.
(2) Increasing the number of the functional groups in order to obtain a strong interaction with carbon black.
(3) Having a reactive portion which produces a strong bonding with the matrix polymer such as a diene rubber.
(4) The reactive portion of (3) is not reactive during the process such as the kneading step, from the viewpoint of processability.
(5) The entire molecular weight being relatively low from the viewpoint of reactivity with carbon black.
Further, the present inventors have discovered an amino-containing low molecular weight diene polymer as a surface-treating agent and have completed the present invention by utilizing this polymer.